Fix a subtle semantic issue with poison values that came up in

Fix a subtle semantic issue with poison values that came up in
recent discussions. Poison can't make every value that depends on
it act in maximally undefined ways, because the optimizer may still
hoist code following the usual rules for undef. Make Poison invoke
its full undefined behavior only when it reaches an instruction with
externally visible side effects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145913 91177308-0d34-0410-b5e6-96231b3b80d8